The field of the invention is water treatment. The invention is particularly useful in the treatment of drinking water, which often contains oxidized contaminants.
Treatment to remove oxidized contaminants is an important step in providing a drinking water supply. Municipal, agricultural and industrial brines, and waste waters must also be treated to remove contaminants before reuse or return to the environment. Nitrate and nitrite are particular common oxidized contaminants that must be removed from drinking water and other waters. Removing nitrate (NO3xe2x88x92) and nitrite (NO2xe2x88x92) from drinking water is becoming increasingly important because of the risk posed to human health from their contamination of groundwater and surface water. Nitrite is a direct agent for methemoglobinemia and cancer, while nitrate is converted to nitrite in the human digestive system. Therefore, the USEPA has set maximum contaminant levels (MCLs) of 10 mg NO3xe2x88x92-N/1 and 1 mg NO2xe2x88x92-N/1.
Denitrification reduces nitrate and nitrite to nitrogen gas (N2). Drinking water typically has very low concentrations of biodegradable organic materials, i.e., it is xe2x80x9coligotrophicxe2x80x9d. Reduction of nitrate (or nitrite) therefore requires addition of an organic (heterotrophic denitrification) or inorganic (autotrophic denitrification) electron (exe2x88x92) donor. In heterotrophic denitrification, ethanol, methanol, and acetate are the most common exe2x88x92donor substrates for drinking water.
Hetertrophic denitrification of drinking water has several disadvantages, which originate from the after-process residuals due to overdosing or variation of influent nitrate concentration. The residual effect can be acutely problematic if the exe2x88x92 donor is harmful. Methanol, for example, has special problems because of its acute toxic effects to humans. In general, organic electron donors are readily biodegradable, and any residual in the water-distribution system promotes excessive microbial growth in the pipelines. The effects of such microbial growth include increased plate counts, foul taste and odor, accelerated pipeline corrosion, and decreased hydraulic capacity of the pipelines. Having residual organic electron donors in the drinking water is called biological instability. Although residuals of inorganic electron donors also would create problems of biological instability, they normally do not persist as a residual. In particular, H2 evolves to the air due to its low water solubility.
In autotrophic denitrification, H2 is also an excellent electron donor choice because of its clean nature and relatively low cost. Denitrification using hydrogen as the electron-donor substrate is called autohydrogenotrophic denitrification, and the H2-oxidizing reaction relies on autotrophs, i.e., bacteria that use an inorganic carbon source. Since the growth rate of autotrophic bacteria is low, good biofilm retention is crucial to prevent the biofilm from washing out from the system.
Previous efforts to conduct autohydrogenotrophic denitrification have used hydrogen gas sparging, either in a separated hydrogen saturation tank or directly to the reactor. See, e.g., Kurt et al. xe2x80x9cBiological Denitrification of Drinking Water Using Autotrophic Organisms with H2 in a Fluidized-Bed Biofilm Reactorxe2x80x9d, Biotechnol Bioeng, 29, 493-501; Tuisel et al. xe2x80x9cBiologische Denitrifikation von Trinkwasser mit Wasserstoff in Einem Fliexcex2bettreaktorxe2x80x9d, G W F Wasser Abwasser, 130:10-13; Dries et al. xe2x80x9cNitrate Removal from Drinking Water by Means of Hydrogenotrophic Denitrifiers in a Polyurethane Carrier Reactorxe2x80x9d, Wat. Supply, 6, 181-192, and Gros et al. xe2x80x9cBiological Denitrification Process with Hydrogen-Oxidizing Bacteria for Drinking Water Treatmentxe2x80x9d, Wat. Supply, 6:193-198. H2-gas sparging of the system results in saturation or close to saturation of dissolved H2 (1.6 mg/l at 20xc2x0 C.) in the water. Therefore, a significant amount of H2 is lost with the effluent stream. This wastes electron donor and might even create an explosive atmosphere under certain conditions.
Thus, there is a need for an improved autohydrogenotrophic water treatment reactor that addresses all or some of the aforementioned drawbacks. It is an object of the invention to provide such an improved reactor.
These and other needs and objects are met or exceeded by the present autohydrogenotrophic water treatment reactor. The present reactor includes a hollow fiber membrane bundle. The membrane bundle is formed from fibers which have microporous inner and outer layers and a nonporous layer sandwiched between the inner and outer layers. Hydrogen is introduced inside the fibers, which are sealed on one end to prevent direct escape of the hydrogen gas. The H2 gas dissolves then diffuses through the nonporous layer. Water is introduced around the fibers, and the bioflim reaction occurs on the outer surface of the fibers. In the biofilm reaction, oxidized contaminants (such as nitrate and nitrite are reduced to harmless products, while the H2 gas is oxidized. The individual fibers are free, over most of their length, to separate in response to the water flow. This prevents excessive biofilm-to-biofilm contact.
A high liquid-phase hydrogen concentration is not required to achieve good removal efficiency in the present reactor. The pressurized hydrogen supply to the fibers allows easy and precise regulation of the hydrogen-supply rate and the liquid-phase hydrogen concentration to achieve target removals. (Partial removal of NO3xe2x88x92, for example, is allowed by the USEPA standards (i.e., the effluent concentration of NO3xe2x88x92 simply needs to be below the standard)). A gentle environment results in the volume around the membrane bundle for stable biofilm accumulation without channelization or clogging.